An electrostatic MEMS switch is a switch operated by an electrostatic charge and manufactured using micro-electro-mechanical systems (MEMS) techniques. A MEMS switch can control electrical, mechanical, or optical signal flow. MEMS switches have typical application to telecommunications, such as DSL switch matrices and cell phones, Automated Testing Equipment (ATE), and other systems that require low cost switches or low-cost, high-density arrays.
As can be appreciated by persons skilled in the art, many types of MEMS switches and related devices can be fabricated by either bulk or surface micromachining techniques. Bulk micromachining generally involves sculpting one or more sides of a substrate to form desired three-dimensional structures and devices in the same substrate material. The substrate is composed of a material that is readily available in bulk form, and thus ordinarily is silicon or glass. Wet and/or dry etching techniques are employed in association with etch masks and etch stops to form the microstructures. Etching is typically performed through the backside of the substrate. The etching technique can generally be either isotropic or anisotropic in nature. Isotropic etching is insensitive to the crystal orientation of the planes of the material being etched (e.g., the etching of silicon by using a nitric acid as the etchant). Anisotropic etchants, such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), and ethylenediamine pyrochatechol (EDP), selectively attack different crystallographic orientations at different rates, and thus can be used to define relatively accurate sidewalls in the etch pits being created. Etch masks and etch stops are used to prevent predetermined regions of the substrate from being etched.
On the other hand, surface micromachining generally involves forming three-dimensional structures by depositing a number of different thin films on the top of a silicon wafer, but without sculpting the wafer itself. The films usually serve as either structural or sacrificial layers. Structural layers are frequently composed of polysilicon, silicon nitride, silicon dioxide, silicon carbide, or aluminum. Sacrificial layers are frequently composed of polysilicon, photoresist material, polyimide, metals, or various kinds of oxides, such as PSG (phosphosilicate glass) and LTO (low-temperature oxide). Successive deposition, etching, and patterning procedures are carried out to arrive at the desired microstructure. In a typical surface micromachining process, a silicon substrate is coated with an isolation layer, and a sacrificial layer is deposited on the coated substrate. Windows are opened in the sacrificial layer, and a structural layer is then deposited and etched. The sacrificial layer is then selectively etched to form a free-standing, movable microstructure such as a beam or a cantilever out of the structural layer. The microstructure is ordinarily anchored to the silicon substrate, and can be designed to be movable in response to an input from an appropriate actuating mechanism.
Many current MEMS switch designs employ a cantilevered beam/plate, or multiple-supported beam/plate geometry. In the case of cantilevered beams, these MEMS switches include a movable, bimaterial beam comprising a structural layer of dielectric material and a layer of metal. Typically, the dielectric material is fixed at one end with respect to the substrate and provides structural support for the beam. The layer of metal is attached on the underside of the dielectric material and forms a movable electrode and a movable contact. The layer of metal can be part of the anchor. The movable beam is actuated in a direction toward the substrate by the application of a voltage difference across the electrode and another electrode attached to the surface of the substrate. The application of the voltage difference to the two electrodes creates an electrostatic field, which pulls the beam towards the substrate. The beam and substrate each have a contact which is separated by an air gap when no voltage is applied, wherein the switch is in the “open” position. When the voltage difference is applied, the beam is pulled to the substrate and the contacts make an electrical connection, wherein the switch is in the “closed” position.
One of the problems that faces current MEMS switches is unwanted contact of the electrode pair. The electrodes of a MEMS switch are ideally positioned very close together while in an “open” position. By placing the electrodes closely together, the power required to deflect the beam to the “closed” position is reduced. However, an unwanted contact of the electrodes can result from this design. The electrodes can also touch if the beam deforms in such a way that the electrodes touch when the beam is moved to the “closed” position. Other undesirable structural deflections usually result from intrinsic or extrinsic stresses in the structural materials. Structural deflections due to intrinsic material stresses occur as a result of a nominal material stress value in combination with the structure design and/or an unbalanced composite structure, or a result of a stress gradient through the thickness of the structural material. The state of nominal and gradient residual stresses is a function of many varied processing conditions and parameters. A common undesirable structural deflection due to extrinsic stress occurs over temperature in composite structures comprised of two or more materials with different Coefficients of Thermal Expansion (CTE). It is undesirable for the electrodes to touch because an electrical short between the electrodes can result.
Some current MEMS switch designs having the bimaterial beam attempt to solve electrode shorting due to beam deformation problems by attaching the metal layer to the topside of the dielectric material. This design serves to prevent electrode shorting during beam deformation; however, this design requires a higher voltage for actuation because the gap distance between the metal layer and the electrode attached to the surface of the substrate is greater. Thus, such a design requires greater power consumption and possibly additional electrical components to achieve higher actuation voltages.
Therefore, it is desirable to provide a beam for improving the yield, performance over temperature, actuation, and quality of MEMS switches. It is also desirable to reduce the likelihood of electrodes touching each other during operation of the switch. It is also desirable to reduce the deformation of the beam in order to improve switch reliability. Furthermore, it is desirable to reduce switch power consumption.